Extension Educator, Horticulture
Once we get a few days of warm weather every gardener turns into Edward Scissorhands. A sure sign of spring are the callused hands from days spent pruning every tree and shrub within armlength. Other than being a rite of spring, pruning can promote new plant growth, maintain plant size, encourage flowering, remove diseased or dead limbs and help control insect and disease problems.
However, timing is important, especially when pruning spring blooming shrubs. Early blooming shrubs develop their flower buds during the summer and fall of the previous year. Therefore as a general rule, shrubs that flower before June 15 should be pruned immediately after flowering. Pruning these shrubs in late summer, fall or early spring will remove the flower buds and therefore the flowers for the season. Spring flowering shrubs are generally pruned by the renewal method. Each spring after flowering, prune out the largest stems to the ground to stimulate new growth from the crown and remaining stems. Shrubs that should be pruned immediately after flowering include lilac, deutzia, kerria, mockorange, weigela, forsythia, arrowwood viburnum, St. John's Wort and red twigged and yellow twigged dogwood. Shrubs that bloom after June 15 can be pruned in early spring, generally February and March. Many of these shrubs can be pruned by the scary rejuvenation method. Rejuvenation is the complete cutting of all stems down to 4 to 6 inch stubs.
Rejuvenation is used when multi-stemmed plants become too large with too many stems to justify saving any one to two year old growth. In other words, the shrub is a tangled mess of stems. However, don't wait too late in the year to do rejuvenation pruning. Especially large old shrubs should not be rejuvenated in late spring or summer. The following respond well to rejuvenation pruning: anthony waterer sprirea, abelia, honeysuckle, beauty bush, snowberry, slender deutzia and privet. Potentilla is another common summer blooming shrub. However, it should be pruned by the renewal method (selectively removing some of the older growth) in early spring just as the green buds appear. Pruning at other times can cause dieback and sunscald, which could kill plants. Before pruning, examine the plant thoroughly. If you are new to pruning, tie a colorful ribbon or spray paint the branches to be removed to help you visualize what will be remaining.
Shrub pruning requires a good set of pruning shears or loppers not hedge shears. At each cut of hedge shears, many new growing points are produced making the shrub very dense at the top. The dense growth causes the leaves in the interior of the plants to die.
Hedge shears should only be used on formal hedges. However, formal hedges must be pruned in the correct shape or the shrubs will lose their leaves at the base. Hedges pruned with perpendicular sides or sides that angle in toward the base will lose the lower leaves. The condition will worsen as the plant matures. If a hedge is already in this condition, it can be rejuvenated by cutting back to 6 to 10 inch stubs in early spring and then reshaping the new growth. If the plants are weak or diseased, plant health should be corrected before rejuvenation. Deciduous shrubs that can be kept as a formal hedge include privet, bush honeysuckle, spirea, pearlbush, ninebark and alpine currant. Deciduous shrubs should not be maintained at a permanent size during the growing season, but allowed to grow an inch or so at each shearing to maintain a good cover of new leaves. Then in February or March, the shrubs can be pruned back to the desired size or smaller.
For more information on pruning, contactyour University of Illinois Extension Office for the circular U5040 Pruning and Care of Trees and Shrubs.